Due to Environment Protection, consumers have attention on electric vehicles. In general, electric vehicle includes hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV or PHV) and battery electric vehicle (BEV). NiMH rechargeable battery is widely applied in HEV because its safety and reliability. But NiMH rechargeable battery has disadvantages, such as small energy storage, large size and battery memory effect. Lithium (Li) ion rechargeable battery has advantages, such as light weight, high energy storage, less self-charge, no battery memory effect and more charge/discharge cycles. Li ion rechargeable battery has potential to replace NiMH rechargeable battery in electric vehicle applications.
Li ion rechargeable battery has high energy density and therefore its safety is an issue. If over-charged, the temperature of the Li ion rechargeable battery is raised and the energy stored therein is too much. Electrolytic solution is decomposed into gas. Its internal atmospheric pressure is higher and the Li ion rechargeable battery will be spontaneous ignition or broken. The estimation of SOC (state of charge) for the rechargeable battery is important.